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The present invention relates to mounting techniques for semiconductor devices and more specifically to a method and system for mounting a lidless semiconductor device.
Semiconductor devices are often mounted to a printed circuit board via a semiconductor device socket. By mounting a semiconductor device in this manner, the device may be readily removed and replaced in the event of a device failure.
Semiconductor device packages take a number of forms. In a lidded semiconductor device, a semiconductor die is mounted to a substrate. A heat spreader plate having a cavity on the underside of the plate that is sized to receive the die is mounted to the substrate with the die positioned within the cavity. Thermal epoxy is underfilled in the cavity surrounding in the area surrounding the die. This structure provides mechanical rigidity for the semiconductor device and allows for heat removal from the die. More specifically, heat removal from the semiconductor die may be accomplished by abutting a heat sink to the top surface of the heat spreader plate. Heat is conveyed from the die to the thermal epoxy and from the thermal epoxy to the heat spreader plate and the abutting heat sink. To obtain efficient heat removal, the heat sink must sufficient force to the top surface of the lidded device to achieve good thermal conductivity.
The above-described technique achieves less than optimal heat removal since thermal epoxy is not an ideal conductor of heat. The failure to adequately remove heat from a semiconductor device can result in the failure of the device.
Recognizing that thermal epoxy impedes heat removal from the semiconductor die, in some systems, lidless semiconductor devices are employed. More specifically, lidless semiconductor devices that have a ball grid array have been soldered directly to a circuit board. A heat sink has been mounted above such devices and pressure has been applied to the heat sink to urge the heat sink against the top surface of the die so as to provide an effective thermally conductive interface between the top surface of the die and the heat sink.
While it is desirable to employ sockets for the mounting of semiconductor devices, the mounting of a lidless semiconductor device such as a land grid array (LGA) device is problematic. A minimum pressure is required to assure proper electrical conductivity between the contacts on the underside of an LGA device and associated conductive contacts in the socket. Considerably less pressure is required to provide proper thermal conductivity between a heat sink and the top surface of a lidless device. The application of forces to the top surface of the lidless device that are sufficient to obtain good electrical conductivity between the semiconductor device contacts and the socket contacts may result in damage to the semiconductor die.
Accordingly it would be desirable to be able to mount a lidless semiconductor device within a socket, such as an LGA socket in a manner that provides the forces needed to assure proper electrical conductivity at the respective interfaces while not subjecting the semi-conductor die to potntial damage as a consequence of expressive forces imported by a heat sink.
A method and apparatus for mounting a lidless semiconductor device is disclosed. A lidless semiconductor device, such as a land grid array device, comprising a substrate having a semiconductor die mounted thereon is disposed in a socket. The socket includes a plurality of conductive members arranged in a predetermined contact array pattern for conductively coupling conductive contacts on a printed circuit board with corresponding contacts on the underside of the lidless semiconductor device. A first set of springs applies pressure to the semiconductor substrate via a pressure plate so as to compress the conductive members of the socket between the printed circuit board and the lower surface of the lidless semiconductor device and conductively couple the contacts of the lidless semiconductor device to corresponding contacts on the printed circuit board. A heat sink is mounted above the socketed lidless semiconductor device and a second set of springs urges a pedestal integral with the bottom surface of a heat sink into abutting relation with the top surface of the semiconductor die. The pressure on the die generated by the second set of springs is sufficiently low to avoid damage to the semiconductor die.
Other features, aspects and advantages of the presently disclosed invention will be apparent from the Detailed Description of the Invention that follows.